Heretofore, in engines mainly for vehicles capable of running with road wheels, such as automobiles, trucks, wheel loaders, and cranes, superchargers have been employed to reduce the size of the engine and to attain a higher output while ensuring satisfactory acceleration properties. Such superchargers include the mechanical type, which utilizes part of an engine output or power from any other means, and the turbo type, which utilizes a turbocharger which uses exhaust gas. The mechanical supercharger is arranged, as shown in FIG. 72, such that a supercharger 210 is mechanically connected to an engine 211 through gears 212, 213, a belt, or the like, for directly driving the engine 211. This direct driving type supercharger 210 produces air supply states as shown in FIG. 73. Specifically, the supply of air to the engine 211 is in a deficient state in a low-speed, high-load region (HA) of the engine rotational speed, and in an excessive state in a high-speed, low-load region (LB). This increases the loss in the driving of the supercharger 210.
To overcome the above drawback, there is proposed a method of driving the supercharger 210 with a differential driving mechanism as shown in FIG. 74 (see Japanese Patent Publication No. 44-1099).
In this proposed differential driving mechanism S1, a planetary carrier 202 is fixedly coupled to an output shaft 211a of an engine 211, and the planetary gears 214, usually three in number, are rotatably mounted on the planetary carrier 202 with equal angular spacings therebetween. The three planetary gears 214 are each held in mesh with a sun gear 215 on the inner side and with a ring gear 216 on the outer side. The output shaft 211a of the engine 211 is disposed to rotatably penetrate the sun gear 215. Further, a gear 217 is provided at one end of the sun gear 215, and a drive gear 218 for the mechanical supercharger 210 is meshed with the gear 217. A shaft, of a power transmission system 220 for a vehicle, is coupled to the ring gear 216, so that the mechanical supercharger 210 is rotated variably responsive to the load of the power transmission system 220.
In the above prior art differential driving mechanism 200, however, because the vehicle load is large during start-up acceleration from zero speed (i.e., acceleration of the vehicle from a stopped state), the vehicle can start moving merely after the rotational speed of the mechanical supercharger 210 has increased to such an extent that a sufficient amount of air is admitted to enter the engine 211. Therefore, the operator has a slow start-up feeling in the initial stage of depressing an accelerator pedal downwardly, and hence is dissatisfied with a poor response.
Also, another differential driving mechanism 201, having a similar arrangement to the above differential driving mechanism 201, is known as shown in FIG. 75 (see, e.g., Japanese Patent Laid-Open No. 5-180282). In the mechanism 201, the ring gear 216 is provided with a brake 216a, which is actuated in an electric, dry air, hydraulic, or other like manner. The force of rotating the mechanical supercharger 210 is controlled responsive to the magnitude of the dragging resistance produced by the brake 216a.
However, the differential driving mechanism 201 has a problem in that part of the driving force of the engine 211 is uselessly consumed as a heat loss through the dragging resistance produced by the brake 216a, etc., resulting in poor fuel economy.
Construction, conveyance, and other machines have different desired output performance of engines, depending on the type to be applied. Some machines are desired to have a higher output at low and medium rotational speeds, and some other machines are desired to have a higher output at high rotational speeds. For example, it is desired for engines of power shovels, etc., to have a higher output at high rotational speeds, i.e., to be operated in a useful region Qa, as shown in FIG. 76. On the contrary, it is desired for engines of bulldozers, dump trucks, wheel loaders, motor graders, on-road trucks, etc., to have a higher output at medium and low rotational speeds, as shown in FIG. 77.
However, there is a problem in that the presently used engines have a characteristic curve indicated by a dashed line EL-1 in FIG. 77, and cannot quickly produce a higher output at medium and low rotational speeds.
This is because of the following problems encountered in the presently used engines:
(1) The amount of intake air is limited in the range of medium and low rotational speeds. PA1 (2) The maximum combustion pressure in a cylinder has a limit at Pmax. PA1 (3) If the compression ratio is reduced at low temperature, start-up properties are deteriorated and hence have a limit. (In view of start-up properties, the compression ratio is approximately 15 to 19 for the case of direct injection.)
FIG. 78 is a graph for explaining the two problems (1) and (2) above. In FIG. 78, the horizontal axis represents an engine rotational speed, the vertical axis represents a net mean effective pressure Pme, and the hyperbolic curves (Ar1, Ar2, Ar3) represent equi-amount lines of air demanded by the engines. Also, a one-dot-chain line (Ara) indicates a limit of the intake amount of air in the engines presently used, and a two-dot-chain line (Pml) indicates a limit of the maximum combustion pressure (Pmax) imposed on the engines presently used from the structural balance in design. From these limits, engines are designed at the present so as to produce an output torque indicated by a dashed line (Tca). While turbosuperchargers are employed to produce a higher output, a prior art turbosupercharger is not operated at a low speed (Na) when the engine has a torque curve (Tca) as shown in FIG. 79. This increases the difference in the demanded air amount to be supplied to the engine between a rated point (Ra) and a maximum torque point (Tmax), and hence requires a compressor map covering a wide range. Therefore, a vaneless type compressor must be used to cover the wide range, resulting in poor compressor efficiency. Further, because of the great difference in the demanded air amount, a maximum efficiency area (Laa), as shown in FIG. 80, cannot be utilized. If the rated point (Ra) is set in the maximum efficiency area (Laa), the maximum torque point (Tmax) would come into an unusable surging region (beyond a surge line Saa) where the air flow rate is too small. On the other hand, if the maximum torque point (Tmax) is set in the maximum efficiency area (Laa), the rated point (Ra) would come into a choke region (beyond a choke line Caa), resulting in the problem of an abnormal reduction in compressor efficiency.
Moreover, if a high-torque engine, capable of producing a high torque at low speeds, is manufactured to include a mechanical supercharger, the pressure in the cylinders would be increased to such an extent as to exceed an allowable limit in design and, accordingly, reliability and durability could not be ensured. If the compression ratio is reduced to overcome the above drawback, the start-up properties at low temperature would be deteriorated. Thus, there arise contradictory problems.
In addition, if the net mean effective pressure is increased in a low-speed range, problems would occur in that a torque at low speeds is remarkably increased to cause a lubrication failure in the main moving portions, or an increased output in the low-speed range causes deficient cooling in the respective portions, whereby durability and reliability are lowered.